Fluid pressure relief system for pressure vessels

ABSTRACT

A fluid pressure relief system and method for a pressure vessel or tank (10) having a normal vent (16) with a first pilot operated safety valve (18) and an emergency vent (62) having a second pilot operated safety valve (25). A vapor recovery line (20) is connected to relief valve (18) and collected at a vapor storage container (21). A manway (22) on the pressure vessel (10) shown in FIG. 2 defines the emergency vent (62) and a main valve member (66) has a counterweight (74) thereon to urge main valve member (66) to a seated closed position on nozzle (60). The first pilot operated safety valve (18) for the normal vent (16) is shown in FIG. 7 and has a T-shaped body (142) with a main relief valve member (172) at one end of the body (142) mounted for movement in a generally horizontal direction and controlled by a diaphragm (176) at an opposite end of the body (142).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of pending application Ser.No. 906,559 filed Jun. 29, 1992.

FIELD OF THE INVENTION

This invention relates to a fluid pressure relief system for pressurevessels, such as tanks, and more particularly to such a fluid pressurerelief system utilizing pilot operated relief valves and the method forproviding the relief system.

BACKGROUND OF THE INVENTION

Heretofore, pilot operated relief valves have been utilized on pressurevessels, such as tanks, such as shown in U.S. Pat. No. 4,705,065 datedNov. 10, 1987, for example, in the opening and closing of a main reliefvalve member and including the opening of the main valve member under avacuum condition. However, the main relief valve has not functionedindependently of the pilot valve such as may be necessary for a severeservice and corrosion environment. The main valve member is in a failopen position in the event of any malfunctioning of the pilot valve.

Present pilot operated relief valves can be operated at pressure verynear the set pressure without leakage, and general seat tightness underall conditions is superior because of the high seating loads. Weightedand spring loaded relief valve members, however, are very simple andprovide consistent reliability for users. Pilot valves which alsoprovide excellent reliability, except under extreme service conditions,are more complex in operation but have superior performance.

Pilot operated relief valves are particularly useful for low pressurevessels designed for a pressure under 15 psi. Such low pressure vesselsare required to have: (1) a normal vent (usually small in orifice size)that limits the pressure rise due to either the addition of liquid tothe vessel or the increase in vessel vapor space operating ambienttemperature, (2) a normal breather (usually small in orifice size) thatlimits the vacuum on the vessel caused by removal of liquid from thevessel or a reduction in vessel vapor space operating ambienttemperature, and (3) an emergency pressure vent (usually large inorifice size) that relieves the pressure in a vessel in the event thetank is subjected to fire or the like. Present code requirements for alow pressure vessel do not permit the setting of a valve above themaximum allowable working pressure (MAWP).Full relief capacity for theemergency vent may be at a pressure of 20% above the MAWP.

Heretofore, pilot operated relief valves have been utilized with thenormal vent of a pressure vessel and upon opening of the main valvemember under control of the pilot, fluid vapors have normally beenvented to atmosphere. Recent requirements of the EnvironmentalProtection Agency (EPA) limit the leakage of harmful vapors frompressure vessels to a maximum concentration of 500 parts per million andthese requirements have to be accomplished by Dec. 31, 1992. Thus, it isnecessary under certain conditions to collect the leakage of any harmfulvapors from the pilot valve, particularly where the pilot valve is setto operate at a set pressure generally around 100% of the MAWP, forexample, and this causes a relatively high frequency of operation of thepilot valve resulting in an opening of the main valve member for vaporleakage.

SUMMARY OF THE INVENTION

The present invention is particularly directed to a fluid pressurerelief system for pressure vessels having (1) a normal vent of arelatively small orifice size to limit a pressure increase resultingfrom the addition of liquid to the vessel or an increase in the ambienttemperature in the vapor space, and (2) a separate emergency vent of arelatively large orifice size that relieves the pressure in the pressurevessel in the event of an emergency such as a fire. A pilot operatedsafety relief valve is utilized with the normal vent and a separatepilot operated safety relief valve is utilized with the emergency vent.

The pilot operated safety relief valve for the emergency vent may beinstalled as a retrofit unit on the manway of a tank with the reliefvalve supported on the manway and extending within the tank from themanway. An emergency override is provided for opening of the main valvemember in the event of any malfunctioning of the pilot valve.

Also, a vacuum vent for the pressure vessel is provided to limit thevacuum on the pressure vessel resulting from removal of liquid from thevessel or a reduction in the operating ambient temperature in the vaporspace of the pressure vessel. The vacuum vent and the emergency vent inthe present invention communicate with the atmosphere when opened butthese vents rarely open except in emergency conditions. However, thenormal pressure vent is connected to a vapor recovery or collectiondevice particularly in view of a low setting of the pilot valvesrelative to the MAWP generally around 98% of the MAWP, and a resultinghigh frequency of operation of the pilot valve.

With the two functions being divided, (1) normal venting to a vaporrecovery means, and (2) emergency venting and vacuum venting to theatmosphere, it is important to provide an efficient and effective systemfor these two functions as in the present invention. The hazardouschemicals in pressure vessels are potentially extremely corrosive. Sincechloride compounds make up a major portion of these chemicals, themixing of the chemicals with atmospheric moisture results inhydrochloric acid, which is destructive to practically all but the veryexpensive metals (such as Hastelloy). The chemicals inside the pressurevessel, however, are usually non-corrosive unless moisture is present inthe system.

A primary purpose of this invention is to provide a new and uniquesystem for the control of vapor release or leakage from pressurevessels. The emergency vent normally operates only under fire conditionsor in the event of failure of the normal vent to function and the vacuumprotection is activated only if the inert gas system using nitrogen witha pressure control regulator should fail. However, because of potentialfugitive emissions to atmosphere, the seats for the emergency and vacuumvents have to be of the highest sealing integrity since any leak becomesan atmospheric leak.

It is an object of this invention to provide a fluid pressure reliefsystem and a method for such system for a pressure vessel having anormal vent and a separate emergency vent with a separate pilot operatedsafety relief valve for each of the vents.

Another object of this invention is to provide such a fluid pressurerelief system in which a manway in the pressure vessel acts as a nozzlefor the emergency vent, and a pilot operated relief valve mounted withinand supported by the manway is operable upon a vacuum condition oremergency pressure condition.

It is an additional object of this invention to provide a pilot operatedrelief valve which may be installed within the manway of an existingtank as a retrofit unit supported by the manway.

It is a further object of this invention to provide such a fluidpressure relief system in which separate pilot operated relief valvesare provided for a normal small diameter vent opening and an emergencylarge diameter vent opening with a vapor recovery device connected tothe normal vent opening to collect vapor therefrom and prevent theescape of possible harmful vapors to atmosphere upon opening of thenormal vent main valve member.

It is another object of this invention to provide a pilot operatedrelief valve for the emergency vent opening in which the main valvemember is capable of opening independently of the pilot valve thereby toprovide reliability and operability in the event of possiblemalfunctioning of the pilot valve.

A further object of this invention is to provide a fluid pressure reliefsystem utilizing pilot operated relief valves for a pressure vesseldesigned for operating fluid pressures under 15 psi and providing aninert gas for the vacuum vent for the pressure vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generally schematic view of a fluid pressure relief systemfor a pressure vessel having a relatively small diameter normal vent anda large diameter emergency vent with a pilot operated relief valve foreach of the vents;

FIG. 2 is an enlarged sectional view of the pilot operated relief valvemounted on the manway of the pressure vessel which forms the largediameter emergency vent with the main valve member in closed seatedposition over the emergency vent;

FIG. 3 is an enlarged cross sectional view of the pilot valve shown inFIG. 2;

FIG. 4 is a section taken generally along line 4--4 of FIG. 3;

FIG. 5 is an enlarged fragment of the pilot valve designated by numeral5 in FIG. 3 showing the valve member in a seated position;

FIG. 6 is a generally schematic sectional view of the pilot operatedrelief valve of FIG. 2 but showing the main valve member in an openposition under an emergency condition opening in response to the pilotvalve;

FIG. 7 is an enlarged cross section view of an embodiment of a pilotoperated relief valve for the small diameter normal vent of the pressurevessel;

FIG. 7A is an enlarged cross sectional view of the connection of themain valve member shown in the embodiment of FIG. 7 to the valve stem;

FIG. 8 is a modified embodiment of a pilot operated relief valve for thenormal vent in which a gas spring from an inert gas source applies aconstant loading for seating of the main valve member;

FIG. 9 is a modified embodiment of a pilot operated relief valve for theemergency vent in which a separate inert gas source is provided to thediaphragm chamber for operation of the main valve member;

FIG. 10 is a modified embodiment of a pilot operated relief valve forthe normal vent in which a spring is provided in the diaphragm chamberfor seating of the main valve; and

FIG. 11 is a further modification of a relief valve for a normal ventwhich is adapted for opening under a vacuum condition and utilized as avery low pressure vacuum breaker with high integrity seats underpositive pressure.

DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a fluid pressure relief system in accordance withthe present invention is illustrated generally schematically. A pressurevessel is shown generally at 10 and comprises a tank preferably having aliquid fluid stored therein with the liquid level shown at 12 and avapor space 14 in the tank above the liquid. Oftentimes, chemicals arestored in the tank and various chlorine compounds which are hazardousand corrosive normally comprise a substantial portion of such chemicals.

A normal vent for tank 10 is shown at 16 and a pilot operated reliefvalve shown generally at 18 is mounted across vent 16. Pilot operatedrelief valve 18 has a set pressure at which to open and often the setpressure is only slightly below the maximum allowable working pressure(MAWP). Thus, the main valve member closing the vent opens with a highfrequency and fluid vapors are normally emitted with each opening. Avapor recovery line 20 is connected to relief valve 18 and receives thevapor upon the opening of the main valve member. The emitted vapors (1)may be burned if inflammable from a flare stack, or (2) may be collectedin a vapor storage chamber or container shown schematically at 21 andreprocessed for return to pressure vessel 10 if desired. Thus, hazardousvapors are not emitted to the atmosphere.

Referring to FIG. 2, pressure vessel 10 has a manway generally indicatedat 22 to permit a workman to enter the tank. Manway 22 includes a manwayopening defined by a tubular housing 23 at least around eighteen (18)inches in diameter projecting upwardly from pressure vessel 10 andgenerally forming an emergency vent. Housing 23 of manway 22 has anoutwardly extending horizontal flange 24 and a separate pilot operatedrelief valve generally indicated at 25 is supported on flange 24 ofmanway 22 and may be installed as a retrofit unit on flange 24 of anexisting tank by suitable nut and bolt combinations 26. Pilot operatedrelief valve 25 comprises an annular mounting plate or ring 28 having aplurality of spaced openings 30 for receiving nut and bolt combinations26 for securing relief valve 25 on flange 24. An upper housing or shroud32 is secured to the upper surface of annular plate 28. A plurality ofrods or struts 34 are secured by suitable fasteners to annular plate 28and extend vertically therefrom. An upper cover 38 is secured by nuts 40to the extending upper ends of rods 34 and a plate 42 is securedadjacent the lower ends of rods 34. Fluid conduits 37 and 39 are securedbetween mounting ring 28 and cover 38 with conduit 37 also secured toplate 42.

A lower operating diaphragm is shown at 44 beneath plate 42 and defininga dome fluid pressure chamber 46 therebetween. Diaphragm 44 is grippedbetween a spacer 48 and lower ring 50 secured to rods 34 and separaterods 51. Conduit 37 forms a dome fluid conduit and provides fluidcommunication between dome chamber 46 and a dome port 48 to a pilotvalve generally indicated at 50 and mounted on cover 38. Conduit 39forms a fluid sensing conduit and has a lower sensing port 56 in fluidcommunication with the interior of tank 10 and an upper inlet port 58 topilot valve 50.

Mounting ring 28 has a separate inner concentric nozzle ring 60 fittingwithin a notch 59 in mounting ring 28 and defining a nozzle. A retainerstrip 61 is received within a slot 63 in nozzle ring 60 and is securedto ring 28 to hold nozzle ring 60 in position. A suitable O-ring seal isprovided between nozzle ring 60 and mounting ring 28. Nozzle ring 60forms an emergency vent port 62 and an upper annular seat 64. A mainpressure relief valve member generally indicated at 66 includes an outerseat ring 68 connected by ribs or spokes 70 to an inner hub 72. Seatring 68 comprises an annular plate which extends radially outward ofnozzle ring 60 to provide an area exposed to fluid pressure upon openingof relief valve 66. An annular counterweight 74 is mounted on seat ring68 to maintain seat ring 68 in seated closed position on annular seat 64under a predetermined constant loading with seat ring 68 moved upwardlyto an unseated open position at a predetermined fluid pressure withintank 10 above the maximum allowable working pressure. A limited flexureof nozzle ring 60 relative to mounting plate may occur fromcounterweight 74. An operating shaft or rod 76 is secured to hub 72 ofvalve member 66 at 78 and to diaphragm 44 at 80. Cross ties 82 aresecured to rod 76 and maintain counterweight 74 in position. Upperhousing 32 about main relief valve member 66 extends a substantialdistance outwardly of main relief valve member 66 and provides a partialrestriction to the vapor or fluid exhausted upon the opening of mainvalve member 66. Such a restriction creates an additional lifting forcefor opening of valve member 66 with a pressure band of around 10%provided between the initial cracking of valve member 66 and the fullopen position of relief valve member 66. Lower plate 42 has a suitableopening receiving rod 76 for sliding movement.

Mounted beneath main valve member 66 for sliding movement on operatingrod 76 is a vacuum valve member generally indicated at 84 having a hub86 receiving rod 76, and a generally concave vacuum plate member 88secured to hub 86 having an outer annular seat 90 adapted to seatagainst the lower surface of main valve member 66. A vacuum spring 92extending about a support tube 94 continuously urges vacuum valve member84 into seated contact against main valve member 66. Upon the reachingof a predetermined low or negative pressure in tank 10, vacuum valve 84moves downwardly along rod 76 to an open position relative to main valvemember 66 to permit an increase in fluid pressure in tank 10 to thepredetermined minimum fluid pressure as controlled by spring 92.

Pilot valve 50 as shown particularly in FIGS. 3-5 has a lower main body96 secured to cover 38. Diaphragm body portions 98 and 100 grip adiaphragm 102 therebetween. A valve 104 is connected to a stem 106secured to diaphragm 102 and movable with diaphragm 102. Valve 104 hasan upper annular extension 108 fitting about an enlarged bulbous end 110of valve 106 with a ball 112 between stem 106 and valve 104 to permitpivoting of valve 104 relative to stem 106. A nozzle 114 is mounted inbody 96 and valve 104 seats against nozzle 114 in a closed position. Theseating force of valve 104 against nozzle 114 is determined by spring116 and adjusting screw 118 acting against the upper end of spring 116to control the compression thereof. A vent to atmosphere is provided at120.

A lower diaphragm 122 secured to shaft 106 separates a fluid inletsensing chamber 124 adjacent diaphragm 102 and a fluid outlet chamber126 vented to atmosphere by vent 128. A fluid passage 129 extendsbetween inlet chamber 124 and inlet port 58. A fluid passage 130 asshown in FIG. 4 extends between dome port 48 and inlet port 58 and inletfluid pressure is in direct fluid communication with dome port 48 anddome chamber 46 through passage 130. A needle valve 132 fitted withinfluid passage 130 provides a variable flow restriction between ports 48and 58 and may be adjusted by an externally threaded screw 134. Needlevalve 132 by varying the flow restriction determines the responsivenessof main valve member 68 to changes in fluid pressure within tank 10 andif a rapid response is desired for opening of main valve member 68 uponan increase in fluid pressure within tank 10, needle valve 132 isadjusted inward a maximum amount. A suitable pilot valve is sold asModel No. 93 by Anderson, Greenwood & Company, Stafford, Tex.

As indicated above, pilot operated relief valve 25 is provided foroperation only in an emergency or negative pressure situation in whichpilot operated pressure relief valve 18 is not adequate for relief ofthe undesired pressure condition. The predetermined high fluid pressureat which relief valve 22 is set to open is greater than the fluidpressure level at which relief valve 18 is set to open.

For operation of pilot operated valve 25, main valve member 68 remainsin seated closed position on nozzle 60 until a predetermined high fluidpressure is reached in tank 10. For example, with pilot valve 50 set at10 psi for a low pressure vessel under 15 psi, pilot valve member 104remains in closed seated position along with main valve member 68 untila predetermined high pressure, such as 10 psi for example, is reachedwithin tank 10. Fluid pressure in inlet sensing chamber 124 from inletport 58 and port 129 moves diaphragm 102 and shaft 106 upwardly tounseat pilot valve member 104 and thereby vent dome chamber toatmosphere through vent 128 to reduce fluid pressure in dome chamber 46and permit opening of main valve member 68. After initial opening orcracking of main valve member 18, if the pressure continues to rise,such as resulting from a fire or the like, main valve member 18 willmove to a fully open position in a relatively short time period.

It is noted that diaphragm 44 has a relatively small opening 136therein. Opening 136 provides a drain for any condensate within domechamber 46 and also provides for an equalization of fluid pressurebetween dome chamber 46 and the interior of tank 10. A time delaydependent on the size of opening 136 is normally provided before thepressure within dome chamber 46 is equalized with the pressure withintank 10. In the event pilot valve 50 malfunctions and pilot valve member104 will not open, main relief valve 25 will override pilot valve 50 andprovide opening of main valve member 68 if a predetermined high fluidpressure is reached in tank 10 such as 11 psi for example.

In the event pilot valve 50 becomes inoperative such as fromobstructions in sensing tubes or conduits 37, 39, or failure ofdiaphragm 102, main relief valve 25 will operate independently. Mainvalve member 66 will crack at a pressure over the set pressure of pilotvalve 50 such as 110% of the set pressure and then move to a fully openposition at a pressure of 120% of the set pressure of pilot valve 50.The 10% pressure band of valve member 66 from crack to full openposition is provided by the additional lifting forces obtained from thepartial restriction provided by shroud or upper housing 32 about valvemember 66. Leakage at valve seat 64 at pressures below the set pressureof pilot valve 50 is prevented as a result of the residual seating loadexerted by counterweight 74.

Referring now to FIG. 7, pilot operated relief valve generally indicatedat 18 is mounted on the normal vent 16 for pressure vessel 10. Vent 16has an upper annular flange 140. Relief valve 18 includes a main body142 formed in a so-called T-section preferably formed of a plasticmaterial such as fiber reinforced polyester. Body 142 has a lower bodyportion 144 extending at right angles to an upper body portion 146.Lower body portion 144 has a lower flange 148 mating with flange 140 andsecured thereto by suitable nut and bolt combinations 150. Upper bodyportion 146 has opposed end flanges 152 and 154 and defines a fluidchamber 156 in continuous fluid communication with the interior ofpressure vessel 10. Vapor recovery line 20 to collection container 21has an annular flange 158 in opposed relation to flange 152 andconnected thereto by suitable nut and bolt combinations 160 to clamp amounting ring 162 therebetween. Mounting ring 162 has a notch 163receiving an inner nozzle ring 164 forming a valve seat 166. Guidemembers 168 are secured to seat ring 162 and extend in a directionparallel to the flow discharge passage 170 defined by valve recoveryline 20. Mounting ring 162 and nozzle ring 164 are similar to thearrangement shown in FIG. 2.

Mounted on valve seat 166 adjacent guide members 168 is the main reliefvalve member generally indicated at 172 secured to one end of anoperating shaft 174. Referring to FIG. 7A, valve member 172 includes anintegral valve disc or valve disc subassembly generally indicated at 167comprising an outer metal plate 169, an adjacent Teflon (a DuPonttrademark) plate or layer 171 preferably formed of a spun Teflonmaterial sold by W. L. Gore & Associates, Inc. under the trademark"Gore-Tex", and a thin Teflon layer or film 173 for contacting valveseat 166. Teflon layer 171 may, for example, be around 0.062 inch inthickness, and Teflon layer 173 may, for example, be around 0.010 inchin thickness. Valve member 172 forms a low pressure seal which seals atpressures between around one-half (1/2) psi and four (4) psi and it isdesired that valve disc 167 seal tightly against valve seat 166. Toensure a tight sealing relation against valve seat 166, disc 167 ismounted on shaft 174 to permit a limited pivoting or swivel motionrelative to shaft 174. Internally threaded collar 175 on shaft 174 hasan internal O-ring 177 sealing against shaft 174. An outer internallythreaded nut 179 on shaft 174 forces spacer 181 and washer 183 againstcollar 175 in a tight gripping action. Disc 167 has a central opening185 of a diameter slightly greater than the outer diameter of spacer 181which is received within opening 185 to permit limited relativemovement. Resilient O-ring 187 is positioned between disc 167 and collar175 while O-ring 189 is positioned between disc 167 and washer 183.Thus, disc or disc subassembly 167 may easily wobble or swivel aboutO-rings 187 and 189 thereby to provide a tight sealing relation betweendisc 167 and valve seat 166. Valve member 66 as shown in FIG. 2 may alsobe provided with a mounting as shown in FIG. 7A, if desired, to providea tight seal against valve seat 64.

A diaphragm 176 is secured to the other end of shaft 174. A partition178 in main body portion 146 supports a hub 180 which receives andsupports shaft 174 for relative sliding movement. A diaphragm domechamber 182 is provided adjacent diaphragm 176. A port 184 extendsbetween dome chamber 182 and fluid chamber 156. An outlet chamber 186 isvented to atmosphere at 188.

A pilot valve is shown at 190 having an inlet line at 192 in fluidcommunication with pressure vessel 10 and dome line 194 extending todome chamber 182. Pilot valve 190 is similar to pilot valve 50 shown inFIGS. 3-5 and operates in a similar manner. A diaphragm 196 is connectedto valve member 198. Diaphragm sensing chamber 200 is in fluidcommunication with inlet line 192. A vent shown at 202 is connected byline 201 to discharge passage 170 to communicate outlet chamber 203 withvapor recovery line 20. Upon an increase in fluid pressure in pressurevessel 10, valve 198 opens to vent the inlet chamber to atmospherethereby to reduce the fluid pressure in dome chamber 182 and move valvemember 172 to an open position. Any vapor or liquid from tank 10 isleaked through line 170 to collection tank 21 for disposal or reuse.Upon a reduction in pressure in tank 10 to a pressure below the setpressure of pilot valve 190, valve member 198 closes and fluid pressurein dome chamber 182 is increased to effect closing of valve member 172.Valve member 172 will normally move back and forth in a modulatingaction to control relatively minor fluctuations in fluid pressure. Inthe event of an emergency and a very high increase in fluid pressurewithin tank such as 103% above the set pressure, valve member 172 willmove to open position and remain in open position. In addition, reliefvalve 25 will open to provide relief in such an emergency situation.Thus, under normal operation and normal fluctuations in fluid pressurein pressure vessel 10 at or just below set pressure, pilot operatedrelief valve 18 is adequate for the relief of pressure within pressurevessel 10. Pilot valve 190 is similar to pilot valve 50 and is sold asModel No. 93 by Anderson, Greenwood & Company, Sugar Land, Texas.

It may be desirable under certain conditions to provide a source ofinert gas, such as nitrogen, for pilot valves 50 and 190. For thatpurpose, a source of inert gas is shown at 183 in FIG. 1 with a pressureregulator 185 in supply line 186 to pilots 50 and 190.

Referring now to FIG. 8 a separate embodiment of the pilot operatedrelief valve shown in FIG. 7 is shown at 18A including a main valvemember 172A connected to one end of shaft 174A and a diaphragm 176Aconnected to the other end of shaft 174A. Dome chamber 182A is suppliedwith fluid from an inert gas source 183A which is preferably nitrogen. Apressure regulator is shown at 185A to control the pressure in dome line194A to dome chamber 182A. Pilot valve 190A has a diaphragm 196A and aninlet sensing chamber 200A in fluid communication with tank 10A frominlet line 192A extending within vent 16A of the associated pressurevessel. Valve member 203A is connected to diaphragm 196A for movementand an outlet chamber 203A is vented to atmosphere through vent 202A.Since the vented fluid is nitrogen or some other inert gas not harmfulto the environment, the vented fluid is not collected. Thus, the fluidin the pressure vessel from vent 16A is isolated from valve member 198Aand dome chamber 182A. Pilot valve 190A functions in a manner similar topilot valve 190 shown in FIG. 7. Upon an increase in fluid pressure inthe associated pressure vessel, an increase in fluid pressure occurs ininlet chamber 200A to open valve member 198A to vent dome chamber 182Ato atmosphere through outlet chamber 203A and vent 202A. A reduction influid pressure in dome chamber 182A effects opening of main relief valvemember 172A until a reduction in pressure occurs in vent 16A thereby toeffect closing of valve member 198A. Safety relief valve 18A wouldnormally be utilized in combination with the emergency safety reliefvalve 22 as shown in FIG. 2.

Referring to FIG. 9, another embodiment of emergency safety relief valveshown in FIG. 2 is shown at 22B. Main relief member 66B is shown withcounterweight 74B and associated vacuum valve member 84B. Main valvemember 66B is seated against seat 64B. Inlet sensing line 39B extendsfrom tank 10B to inlet sensing chamber 200B of pilot valve 190B adjacentdiaphragm 196B. Pilot valve 190B is similar to pilot valve 190A of theembodiment of FIG. 8. Dome line 194B extends to dome chamber 46Badjacent diaphragm 44B connected by shaft 76B to main relief member 66B.An inert gas such as nitrogen is supplied to dome chamber 46B from asuitable gas source shown at 183B and line 186B to fluid pressureregulator shown at 185B, and then through line 187A to pilot valve 190Band dome line 194B.

A back pressure regulator valve is shown at 189B and is in fluidcommunication with dome chamber 46B through line 191B to dome line 194B.Line 191B is in fluid communication with inert gas line 187A throughpilot valve 190B. Regulator 185B supplies inert gas preferably nitrogento pilot 190B through line 187A and conduit 194B to chamber 46B. Theregulated pressure is at a value sufficient to force the main operatingelement downward to seat valve 64B. The pressure is approximately twotimes the pressure in tank 10B in order to keep the valve closed whenthe pressure in tank 10B is less than around 110% of the set pressure ofpilot 190B. If tank pressure rises to the set pressure of pilot 190Bthen fluid pressure acts on diaphragm 200B to open pilot 190B. Thepressure is then reduced in chamber 46B by venting the inert gas toatmosphere. Back pressure regulator valve 189B is similar in function topilot 190B but is set to open at a higher fluid pressure than is valve189B. Its purpose is to set an upper limit on the pressure in chamber46B and to maintain that maximum valve should regulator 185B fail tocontrol the source pressure.

Regulator 185B is of a type commercially available and includes a largediameter diaphragm to sense the low pressure required. The systempressure is sensed through a regulator exhaust port. When the systempressure is below the predetermined set pressure, the regulator opensand allows inert gas such as nitrogen to flow from a fluid supply portto an exhaust port for increasing the system pressure. When the systempressure reaches the predetermined set pressure, the regulator closesand prevents any further increase in the system pressure. A suitablecommercially available regulator is sold as a model RA regulator byAnderson, Greenwood & Company, Stafford, Texas.

Referring now to FIG. 10, another embodiment of the present invention isshown which is adapted for use without being employed with an emergencysafety relief valve such as shown at 22 for the embodiment of FIGS. 1-7.Safety relief valve member 18C is generally similar to the safety reliefvalve member shown in FIGS. 3-5 and includes a T-shaped main body 142Chaving a main valve member 172C therein connected to one end of shaft174C. Diaphragm 176C is connected to the other end of shaft 174C. A domechamber 182C is on one side of diaphragm 176C and diaphragm chamber 186Cis on the other side of diaphragm 176C. Dome line 194C extends to pilotvalve 190C. Diaphragm 196C is connected to pilot valve member 198C anddiaphragm chamber 200C is in fluid communication with the tank pressureat vent 16C by conduit 192C similar to pilot 50 on FIG. 2. Outletchamber 203C is in communication with vapor recovery line 20 throughline 201C. Inlet sensing line 192C has one end in fluid communicationwith vent 16C and its other end in fluid communication with dome line194C and dome chamber 182C. A branch line 193C is in fluid communicationwith diaphragm chamber 186C. When pilot valve member 198C is below theset pressure, the fluid pressure in chambers 182C and 186C is equalized.A dome chamber spring 195C urges main valve member 172C to closedposition against seat 166C with a force higher than the force exertedagainst valve member 172C by the fluid pressure within the pressurevessel at vent 16C. A condensation drain port 184C is provided for domechamber 182C.

Upon an increase in fluid pressure within the pressure vessel above theset pressure, pilot valve member 198C will unseat as a result of fluidpressure from sensing line 192C to reduce the fluid pressure in domechamber 182C with fluid pressure in valve chamber 156C and outerdiaphragm chamber 186C overcoming spring 195C to open main valve member172C. In the event of an emergency condition, such as the malfunctioningof pilot valve 190C, the main valve member 172C will open independentlyand relieve the pressure at some percentage above MAWP such as 105% ofMAWP with a full open flow at 110% of MAWP. The tank fluid pressure atvent 16C will overcome the spring load from spring 195C to open valve172C since the diaphragm chambers 182C and 186C are balanced.

Referring now to FIG. 11, a separate embodiment of the pilot operatedrelief valve shown in FIG. 5 is shown at 18D including a main valvemember 172D connected to one end of shaft 174D and a diaphragm 176Dconnected to the other end of shaft 174D. Dome chambers 182D and 186Dare reversed in location from FIG. 5. Dome chamber 182D is in directcommunication with the fluid in the pressure vessel from vent 16D. Domechamber 186D is in direct communication with the atmosphere and isisolated from the fluid in the pressure vessel by seals in bushing 180Dand diaphragm 176D. Upon an increase in fluid pressure in the pressurevessel, valve member 172D is urged against valve seat 166D by acombination of fluid pressure acting on valve member 172D and diaphragm176D. Upon a decrease in fluid pressure in the pressure vessel to belowatmospheric, the atmospheric pressure force on the valve member 172D anddiaphragm 176D will unseat valve member 172D from valve seat 166Dallowing atmospheric pressure to enter the pressure vessel and preventcollapse.

While not specifically shown in the embodiments of FIGS. 8, 10, and 11,the main valve member 172A, 172C and 172D may be connected to the stem174A, 174C and 174D in a manner similar to that shown in FIG. 7A formain valve member 172 on valve stem 174.

While preferred embodiments of the present invention have beenillustrated in detail, it is apparent that modifications and adaptationsof the preferred embodiment will occur to those skilled in the art.However, it is to be expressly understood that such modifications andadaptations are within the spirit and scope of the present invention asset forth in the following claims.

What is claimed is:
 1. In a pressure vessel having a vent therein forthe relief of fluid pressure within the vessel;an annular fixed nozzledefining said vent; a valve disc subassembly for seating against saidnozzle and having a central opening therethrough; a stem connected atone end to said valve disc subassembly for longitudinal movementtherewith between open and closed positions of said valve discsubassembly relative to said nozzle; and mounting means for mountingsaid valve disc subassembly on said stem for limited pivotal movementrelative to said stem and relative to said nozzle for effecting sealingbetween said valve disc subassembly and said nozzle; said mounting meansincluding: an annular spacer of a generally cylindrical shape about saidshaft having an outer diameter less than the diameter of said centralopening in said valve disc subassembly and fitting within said centralopening; a pair of opposed facing surfaces adjacent opposed ends of saidspacer and extending radially outward of said spacer; an elastomericring between each facing surface and said disc subassembly to space saiddisc subassembly from said facing surfaces to permit relative cushionedwobbling movement of said disc subassembly, the elastomeric rings beingpositioned about and in contact relation with said spacer to permit alimited pivotal movement of said disc subassembly relative to saidspacer and said shaft thereby to effect sealing contact between saidvalve disc subassembly and said nozzle.
 2. In a pressure vessel as setforth in claim 1;said facing surfaces defined by internally threadedmembers engaging external threads on said shaft for securing saidspacer.
 3. In a pressure vessel as set forth in claim 1;said elastomericring between each facing surface and said disc subassembly comprising anO-ring.
 4. In a pressure vessel as set forth in claim 1;a mounting platehaving a central opening and a notch adjacent said central opening, saidnozzle including a nozzle ring fitting within said notch in saidmounting plate, and means to secure said nozzle ring to said mountingplate.
 5. In a pressure vessel as set forth in claim 1;a diaphragmsecured to the opposite end of said stem and controlling the movement ofsaid valved disc subassembly between open and closed positions relativeto said nozzle.